Image charge screening: A new approach to enhance magnetic ordering temperatures in ultrathin correlated oxide films

We have tested the concept of image charge screening as a new approach to restore magnetic ordering temperatures and superexchange interactions in correlated oxide ultrathin films. Using a three-monolayer NiO(100) film grown on Ag(100) and an identically thin film on MgO(100) as model systems, we observed that the Néel temperature of the NiO film on the highly polarizable metal substrate is 390 K while that of the film on the poorly polarizable insulator substrate is below 40 K. This demonstrates that screening by highly polarizable media may point to a practical way toward designing strongly correlated oxide nanostructures with greatly improved magnetic properties.

Figure 1

(Color online) Ag/NiO/Ag cross sectional lamella: (a) SEM image; (b) HRTEM image; (c) HRTEM intensity line scan through Ag/NiO/Ag interfaces; (d) SAD pattern. (e) STM image of uncapped 3 ML NiO(100)/Ag(100) at $V=3\text{V}$ and $I=0.1\text{nA}$. (f) STM intensity line scan along the line in panel (e).

Figure 2

(Color online) Temperature-dependent $\text{Ni}{L}_2$ XAS spectra with linearly polarized light at $\theta =0°$ on 20 ML MgO(100)/30 ML NiO(100)/Ag(100) and related changes in magnetic ordering.

Figure 3

(Color online) Temperature dependent $\text{Ni}{L}_2$ ratio of (a) 30 ML NiO(100)/Ag(100) and (b) 3 ML NiO(100)/Ag(100) (red markers) and 3 ML NiO(100)/MgO(100) (blue markers).

Figure 4

(Color online) Temperature dependent $\text{Ni}{L}_2$ XAS spectra on [(a) and (b)] 25 ML MgO(100)/3 ML NiO(100)/Ag(100) and [(c) and (d)] 25 ML MgO(100)/3 ML NiO(100)/MgO(100) films.